A description is made for a switching device disclosed in Japanese Patent Unexamined Publication No. 2000-207988 using FIG. 10.
FIG. 10 illustrates a structure of a switch on a keyboard used for an input device of an electronic device. Pressing key top 101 opens and closes the switch. This key top 101 is retained by link mechanism 102 so that key top 101 is vertically movable. This link mechanism 102 is connected to extension coil spring 103 made of a shape-memory alloy and biasing member 104 made of an elastic body, where key top 101 is usually biased downward by these members. Extension coil spring 103 made of shape-memory alloy is electrically connected to circuit board 105, via stretchable current-carrying wire 106. Supplying electric power from the circuit causes extension coil spring 103 made of shape-memory alloy to contract due to the shape-memory effect, against the force of biasing member 104, actuating link mechanism 102, and then moving up key top 101.
However, the switching device in the above-mentioned conventional example, where extension coil spring 103 made of shape-memory alloy is connected to the circuit for supplying electric power via stretchable current-carrying wire 106, has a problem in that coupling this current-carrying wire 106 with extension coil spring 103 is difficult. Also, there is a problem in that a resistance caused by deformation of current-carrying wire 106 reduces a generated force by the shape-memory effect of extension coil spring 103. Further, as another problem, the coupling part of current-carrying wire 106 and extension coil spring 103 moves in response to stretching and contraction of extension coil spring 103, and thus the coupling part breaks due to repeated stresses applied to the coupling part, resulting in a loss of reliability of the switching device.